Obvious I had to enter competition (7)
I believe the answer is:
evident
'obvious' is the definition.
(similar in meaning)
'i had to enter competition' is the wordplay.
'i had' becomes 'i'd' (contraction).
'to enter' means one lot of letters goes inside another.
'competition' becomes 'event' (sporting event).
'id' going into 'event' is 'EVIDENT'.
(Other definitions for evident that I've seen before include "Plain to see" , "Clearly apparent" , "'Plain, apparent (7)'" , "Readily apparent" , "Easy to understand" .)